Avoid calling lib_func without a prototype.

This commit mirrors the change in commit f4c0a750d4adebcf2342a44e85f04526c34
("WAF: Fix detection of linker features")
to buildtools/wafsamba/samba_conftests.py.  It fixes the check for rpath
support with compilers in strict C99 mode.

Submitted upstream: <https://gitlab.com/samba-team/samba/-/merge_requests/2807>

diff --git a/buildtools/wafsamba/samba_waf18.py b/buildtools/wafsamba/samba_waf18.py
index e2a078bd3a05fd78..cfdceea14ca4b706 100644
--- a/buildtools/wafsamba/samba_waf18.py
+++ b/buildtools/wafsamba/samba_waf18.py
@@ -209,7 +209,8 @@ def CHECK_LIBRARY_SUPPORT(conf, rpath=False, version_script=False, msg=None):
         lib_node.parent.mkdir()
         lib_node.write('int lib_func(void) { return 42; }\n', 'w')
         main_node = bld.srcnode.make_node('main.c')
-        main_node.write('int main(void) {return !(lib_func() == 42);}', 'w')
+        main_node.write('int lib_func(void);\n'
+                        'int main(void) {return !(lib_func() == 42);}', 'w')
         linkflags = []
         if version_script:
             script = bld.srcnode.make_node('ldscript')
